Light emitting element driving device

ABSTRACT

A light emitting element driving device, which flashing-drive a plurality of light emitting element arrays connected in parallel by switching elements connected in series with each of the plurality of light emitting element arrays, the light emitting element driving device including: a current detection unit configured to detect respective currents flowing through each of the plurality of light emitting element arrays as currents of the light emitting element arrays; a selection unit configured to select the smallest current of the light emitting element arrays of the detected currents obtained by the current detection unit; and an output voltage control unit configured to control output voltage supplied to the plurality of light emitting element arrays so that the current of the light emitting element arrays selected by the selection unit becomes a preset reference current value.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from Japanese Patent Application No.2012-002867 filed on Jan. 11, 2012 the entire subject matter of which isincorporated herein by reference.

TECHNICAL FIELD

This disclosure relates to a light emitting element driving device fordriving a LED (Light Emitting Diode) which is a light emitting element.

BACKGROUND

A light emitting element driving device is known in which a plurality oflight emitting element arrays respectively configured by a plurality ofLEDs connected in series are connected in parallel and the plurality oflight emitting element arrays are driven by constant current driversrespectively connected in series with each of the light emitting elementarrays (for example, see JP-A-2003-332624). In this light emittingelement driving device, the lowest voltage of the voltage applied to aplurality of constant current drivers is selected as a detectionvoltage, and then an output voltage of a power supply device isautomatically controlled so that the value of the detected voltage is areference voltage.

SUMMARY

However, in the background art, it is necessary to set the lowestvoltage of the voltage applied to a plurality of constant currentdrivers to a voltage sufficient to allow the current required to emitlight to flow through the light emitting element arrays. Accordingly, anexcessive voltage is applied to the plurality of constant currentdrivers and thus the power loss is caused in the constant currentdrivers.

With considering the above, this disclosure provides a light emittingelement driving device capable of reducing power loss in the switchingelements to flashing-drive the light emitting element arrays.

A light emitting element driving device of this disclosureflashing-drives a plurality of light emitting element arrays connectedin parallel by switching elements connected in series with each of theplurality of light emitting element arrays. The light emitting elementdriving device comprises: a current detection unit configured to detectrespective currents flowing through each of the plurality of lightemitting element arrays as currents of the light emitting elementarrays; a selection unit configured to select the smallest current ofthe light emitting element arrays of the detected currents obtained bythe current detection unit; and an output voltage control unitconfigured to control output voltage supplied to the plurality of lightemitting element arrays so that the current of the light emittingelement arrays selected by the selection unit becomes a preset referencecurrent value.

Additionally, the above described light emitting element driving devicemay be includes an average current control unit configured to controleach of the duty ratios to drive the plurality of the switching elementsto be turned on/off so that the average value of the current of theplurality of light emitting element arrays detected by the currentdetection unit is equal to each other.

Additionally, in the above described light emitting element drivingdevice, in an ON-period of the switching elements in which any one ofthe plurality of the switching element is driven to be turned on, theoutput voltage control unit may control the output voltage based on thecurrent of the light emitting element arrays, and, in an OFF-period ofthe switching elements in which all of the plurality of the switchingelements are controlled to be turned off, the output voltage controlunit may control the output voltage based on the current of the lightemitting element arrays in the last ON-period of the switching element.

In the above described the light emitting element driving device, theoutput voltage control unit may be configured to generate a referencevoltage correlated to the current of the light emitting element arraysselected by the selection unit and to perform a feedback control of theoutput voltage based on the reference voltage.

According to this disclosure, the light emitting element driving deviceis configured to control the output voltage supplied to the plurality oflight emitting element arrays so that the smallest current of thecurrent flowing through each of the plurality of light emitting elementarrays is a preset reference current value. Accordingly, it is notnecessary to use a constant-current driver in order to flashing-drivethe light emitting element arrays and thus it is possible to reducepower loss in the switching elements that flashing-drive the lightemitting element arrays.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and additional features and characteristics of thisdisclosure will become more apparent from the following detaileddescriptions considered with the reference to the accompanying drawings,wherein:

FIG. 1 is a circuit diagram illustrating a circuit configuration of alight emitting element driving device according to an illustrativeembodiment of this disclosure;

FIG. 2 is a circuit diagram illustrating a circuit configuration of aselection circuit shown in FIG. 1;

FIG. 3 is a circuit diagram illustrating a circuit configuration of anaverage current control circuit shown in FIG. 1;

FIG. 4 is a circuit diagram illustrating a circuit configuration of anoutput voltage control circuit shown in FIG. 1; and

FIG. 5 is a circuit diagram illustrating another circuit configurationof the output voltage control circuit shown in FIG. 1.

DETAILED DESCRIPTION

Referring to FIG. 1, a light emitting element driving device in thepresent illustrative embodiment includes a light emitting unit 1, apower supply device 2 supplying an output voltage Vout to the lightemitting unit 1, and a control device 3 performing a control of theoutput voltage Vout supplied to the light emitting unit 1 and a lightingcontrol of the light emitting unit 1.

The light emitting unit 1 includes light emitting element arrays 11 to13, each array having four LEDs connected in series. Although the lightemitting unit 1 is configured to include three light emitting elementarrays 11 to 13 in the present illustrative embodiment, the number ofthe light emitting element arrays 11 to 13 or the number of the LEDsconnected in series to configure the light emitting element arrays 11 to13 is not particularly limited, but can be suitably set on the basis ofthe required amount of light, etc. Further, although the same ones areused as the light emitting element arrays 11 to 13, some degree ofvariation in characteristics is assumed.

The power supply device 2 in the present illustrative embodiment is aboost chopper circuit and includes a choke coil L1, a diode D1, asmoothing capacitor C1 and a MOSFET Q1. A series circuit configured bythe choke coil L1 and the MOSFET Q1 acting as a switching element isconnected between an input terminal T1 connected to a positive terminalof DC power supply Vin and an earth terminal. Further, a series circuitconfigured by the diode D1 as an output rectifier element and thesmoothing capacitor C1 is connected between a connection point of thechoke coil L1 and the MOSFET Q1 and the earth terminal. Anode-sideterminals of the light emitting element arrays 11 to 13 are connected toa connection point of the diode D1 and the smoothing capacitor C1, andthe output voltage Vout is boosted by the on-off control of the MOSFETQ1 and supplied to the light emitting element arrays 11 to 13.

The control device 3 includes MOSFETs Q2 to Q4, resistors R1 to R5, aselection circuit 4, an average current control circuit 5 and an outputvoltage control circuit 6. A series circuit configured by the MOSFET Q2to flashing-drive the light emitting element array 11 and the resistorR1 is connected between a cathode-side terminal of the light emittingelement array 11 and the earth terminal. Similarly, a series circuitconfigured by the MOSFET Q3 to flashing-drive the light emitting elementarray 12 and the resistor R2 is connected between a cathode-sideterminal of the light emitting element array 12 and the earth terminal,and a series circuit configured by the MOSFET Q4 to flashing-drive thelight emitting element array 13 and the resistor R3 is connected betweena cathode-side terminal of the light emitting element array 13 and theearth terminal. The MOSFETs Q2 to Q4 have the same properties and theresistors R1 to R3 have the same resistance value.

The selection circuit 4 selects the smallest current value of thecurrents flowing through the MOSFETs Q2 to Q4 and outputs a detectedvoltage Vmin corresponding to the selected current value to the outputvoltage control circuit 6. Referring to FIG. 2, a series circuitconfigured by a constant current source CC1, a P-type MOSFET Q10 and anN-type MOSFET Q11 is connected between a control power supply Reg andthe earth terminal. Further, a series circuit configured by a P-typeMOSFET Q12 and an N-type MOSFET Q13 is connected between the controlpower supply Reg and the earth terminal and also a series circuitconfigured by a P-type MOSFET Q14 and an N-type MOSFET Q15 is connectedtherebetween. Furthermore, a parallel circuit configured by P-typeMOSFETs Q16 to Q18 is connected to the control power supply Reg via theconstant current source CC1 and connected to the earth terminal via anN-type MOSFET Q19.

The N-type MOSFET Q19 and the N-type MOSFET Q15 are configured to form acurrent mirror circuit. A gate terminal of the N-type MOSFET Q19 and agate terminal of the N-type MOSFET Q15 are connected to each other and aconnection point thereof is connected to a drain terminal of the N-typeMOSFET Q19. Further, the N-type MOSFET Q11 and the N-type MOSFET Q13 areconfigured to form a current mirror circuit. A gate terminal of theN-type MOSFET Q11 and a gate terminal of the N-type MOSFET Q13 areconnected to each other and a connection point thereof is connected to adrain terminal of the N-type MOSFET Q11. Furthermore, the P-type MOSFETQ12 and the P-type MOSFET Q14 are configured to form a current mirrorcircuit. A gate terminal of the P-type MOSFET Q12 and a gate terminal ofthe P-type MOSFET Q14 are connected to each other and a connection pointthereof is connected to a source terminal of the P-type MOSFET Q12.

A terminal T2 connected to a gate terminal of the P-type MOSFET Q16 isconnected to a connection point of the MOSFET Q2 to flashing-drive thelight emitting element array 11 and the resistor R1. The resistor R1 isa current detection resistor for detecting the current flowing throughthe MOSFET Q2 (the light emitting element array 11) and a voltagecorrelated to the current flowing through the MOSFET Q2 is applied tothe gate terminal of the P-type MOSFET Q16. Further, a terminal T3connected to a gate terminal of the P-type MOSFET Q17 is connected to aconnection point of the MOSFET Q3 to flashing-drive the light emittingelement array 12 and the resistor R2. The resistor R2 is a currentdetection resistor for detecting the current flowing through the MOSFETQ3 (the light emitting element array 12) and a voltage correlated to thecurrent flowing through the MOSFET Q3 is applied to the gate terminal ofthe P-type MOSFET Q17. Furthermore, a terminal T4 connected to a gateterminal of the P-type MOSFET Q18 is connected to a connection point ofthe MOSFET Q4 to flashing-drive the light emitting element array 13 andthe resistor R3. The resistor R3 is a current detection resistor fordetecting the current flowing through the MOSFET Q4 (the light emittingelement array 13) and a voltage correlated to the current flowingthrough the MOSFET Q4 is applied to the gate terminal of the P-typeMOSFET Q18. In addition, a connection point of the P-type MOSFET Q14 andthe N-type MOSFET Q15 is connected to a gate terminal of the P-typeMOSFET Q10 and an output terminal T5.

FIG. 2 shows the selection circuit 4 in a state where a total current Iaflowing through the P-type MOSFETs Q16, Q17 and Q18 flows through theN-type MOSFET Q19 and the N-type MOSFET Q15. Here, a current Ib flowingthrough the P-type MOSFET Q10 flows through the current mirror circuitconfigured by the P-type MOSFET Q12 and the P-type MOSFET Q14 via thecurrent mirror circuit configured by the N-type MOSFET Q11 and theN-type MOSFET Q13. Accordingly, a voltage signal output from the outputterminal T5 becomes a difference signal between the current Ia and thecurrent Ib. However, since the gate terminal of the P-type MOSFET Q10 isconnected to the output terminal T5, the gate terminal is operated tocause the current Ia and the current Ib to be equal. Thereby, in theselection circuit 4 shown in FIG. 2, the lowest voltage is selected ofthe voltage (voltage correlated to the current flowing through theMOSFETs Q2 to Q4) applied to the terminals T2 to T4 and the selectedvoltage is output as the detected voltage Vmin from the terminal T5 tothe output voltage control circuit 6 via a voltage follower using anoperational amplifier.

Referring to FIG. 3, the average current control circuit 5 includes anoscillation circuit 51, detection circuits 52 a to 52 c, integrationcircuits 53 a to 53 c, RS-type flip-flop circuits 54 a to 54 c, drivecircuits 55 a to 55 c and an OR circuit 56.

An input terminal of the oscillation circuit 51 is connected to aterminal T6 to which a dimming signal is input from the outside, and anoutput terminal of the oscillation circuit 51 is connected to the setterminals S of the flip-flop circuits 54 a to 54 c. The dimming signalinput from the terminal T6 is a signal for controlling the brightness ofthe light emitting unit 1 by a duty ratio (time ratio). The oscillationcircuit 51 performs an oscillation operation when the dimming signal isin an ON-duty state and outputs a set signal to set the flip-flopcircuits 54 a to 54 c at a predetermined period.

An input terminal of the detection circuit 52 a is connected to aconnection point of the MOSFET Q2 and the resistor R1 and an outputterminal of the detection circuit 52 a is connected to an input terminalof the integration circuit 53 a. An output terminal of the integrationcircuit 53 a is connected to a reset terminal R of the flip-flop circuit54 a. The detection circuit 52 a detects the current flowing through theMOSFET Q2 (the light emitting element array 11). The integration circuit53 a integrates the current detected by the detection circuit 52 a. Theintegration circuit 53 a outputs a reset signal to reset the flip-flopcircuit 54 a when the current integrated value reaches a predeterminedreference integrated value. Further, an output terminal Q of theflip-flop circuit 54 a is connected to an input terminal of the drivecircuit 55 a. Then, an output terminal of the drive circuit 55 a isconnected to a gate terminal of the MOSFET Q2. The output terminal Q ofthe flip-flop circuit 54 a outputs an on/off signal, which is turned onby a set signal from the oscillation circuit 51 and turned off by areset signal from the integration circuit 53 a. The drive circuit 55 adrives the MOSFET Q2 to be turned on/off based on the on/off signal fromthe flip-flop circuit 54 a.

An input terminal of the detection circuit 52 b is connected to aconnection point of the MOSFET Q3 and the resistor R2 and an outputterminal of the detection circuit 52 b is connected to an input terminalof the integration circuit 53 b. An output terminal of the integrationcircuit 53 b is connected to a reset terminal R of the flip-flop circuit54 b. The detection circuit 52 b detects the current flowing through theMOSFET Q3 (the light emitting element array 12). The integration circuit53 b integrates the current detected by the detection circuit 52 b. Theintegration circuit 53 b outputs a reset signal to reset the flip-flopcircuit 54 b when the current integrated value reaches a predeterminedreference integrated value. Further, an output terminal Q of theflip-flop circuit 54 b is connected to an input terminal of the drivecircuit 55 b. Then, an output terminal of the drive circuit 55 b isconnected to a gate terminal of the MOSFET Q3. The output terminal Q ofthe flip-flop circuit 54 b outputs an on/off signal, which is turned onby a set signal from the oscillation circuit 51 and turned off by areset signal from the integration circuit 53 b. The drive circuit 55 bdrives the MOSFET Q3 to be turned on/off based on the on/off signal fromthe flip-flop circuit 54 b.

An input terminal of the detection circuit 52 c is connected to aconnection point of the MOSFET Q4, and the resistor R3 and an outputterminal of the detection circuit 52 c is connected to an input terminalof the integration circuit 53 c. An output terminal of the integrationcircuit 53 c is connected to a reset terminal R of the flip-flop circuit54 c. The detection circuit 52 c detects the current flowing through theMOSFET Q4 (the light emitting element array 13). The integration circuit53 c integrates the current detected by the detection circuit 52 c. Theintegration circuit 53 c outputs a reset signal to reset the flip-flopcircuit 54 c when the current integrated value reaches a predeterminedreference integrated value. Further, an output terminal Q of theflip-flop circuit 54 c is connected to an input terminal of the drivecircuit 55 c. Then, an output terminal of the drive circuit 55 c isconnected to a gate terminal of the MOSFET Q4. The output terminal Q ofthe flip-flop circuit 54 c outputs an on/off signal, which is turned onby a set signal from the oscillation circuit 51 and turned off by areset signal from the integration circuit 53 c. The drive circuit 55 cdrives the MOSFET Q4 to be turned on/off based on the on/off signal fromthe flip-flop circuit 54 c.

When the dimming signal input from the terminal T6 is in the ON-dutystate, the flip-flop circuits 54 a to 54 c are simultaneously set by theset signal from the oscillation circuit 51 and the MOSFETs Q2 to Q4 aresimultaneously driven to be turned on. As the MOSFETs Q2 to Q4 arecontrolled to be turned on, the current is flowing through the lightemitting element arrays 11 to 13 and respectively detected by thedetection circuits 52 a to 52 c. The detected current is respectivelyintegrated by the integration circuits 53 a to 53 c. When the currentintegrated value in the integration circuits 53 a to 53 c reaches apredetermined reference integrated value (common in the integrationcircuits 53 a to 53 c), the flip-flop circuits 54 a to 54 c arerespectively reset, the MOSFETs Q2 to Q4 are controlled to be turnedoff, and then the current flowing through the light emitting elementarrays 11 to 13 is interrupted. That is, the timing when driving theMOSFETs Q2 to Q4 to be turned off is different in the light emittingelement arrays 11 to 13 in accordance with the current flowing throughthe light emitting element arrays 11 to 13. A duty ratio for driving aplurality of MOSFETs Q2 to Q4 to be turned on/off is controlled so thatthe average value of the current flowing through the light emittingelement arrays 11 to 13 is equal to each other. According to thiscontrol, the average value of the current flowing through the lightemitting element arrays 11 to 13 is equal to each other. Accordingly,even when the value of the current actually flowing through the lightemitting element arrays 11 to 13 is different from each other due tovariation in the characteristics of the light emitting element arrays 11to 13, light can be emitted from the light emitting element arrays 11 to13 at the same brightness.

Further, each of the output terminals Q of the flip-flop circuits 54 ato 54 c are respectively connected to an input terminal of the ORcircuit 56. An output terminal of the OR circuit 56 is connected to theoutput voltage control circuit 6 and SH (Sample Hold) signal is outputfrom the OR circuit 56 to the output voltage control circuit 6. In theSH signal output from the OR circuit 56, a period when on-signal isoutput from any one of the flip-flop circuits 54 a to 54 c, that is, anON-period of a switching element, in which any one of the MOSFETs Q2 toQ4 is driven to be turned on, is referred to as a sample period, and aperiod when off-signal is output from all of the flip-flop circuits 54 ato 54 c, that is, an OFF-period of a switching element, in which all ofthe MOSFETs Q2 to Q4 are controlled to be turned off, is referred to asa hold period.

Referring to FIG. 4, the output voltage control circuit 6 includes OTA(Operational Transconductance Amplifier) error amplifier circuit 61, asample-and-hold circuit 62, an error amplifier circuit 63, a controlcircuit 64 and a reference voltage Vref.

The reference voltage Vref is applied to a non-inverting input terminalof the OTA error amplifier circuit 61 and the detected voltage Vmin fromthe selection circuit 4 is input to an inverting input terminal of theOTA error amplifier circuit 61. The OTA error amplifier circuit 61 is acircuit, which compares the reference voltage Vref and the detectedvoltage Vmin and outputs an error signal therebetween. From an outputterminal of the OTA error amplifier circuit 61, an error currentcorresponding to an error voltage between the reference voltage Vref andthe detected voltage Vmin is output as the error signal.

The SH signal from the average current control circuit 5 is input to acontrol terminal of the sample-and-hold circuit 62. During the sampleperiod of the SH signal, the sample-and-hold circuit 62 detects anoutput current from the OTA error amplifier circuit 61 and outputs avariable reference voltage Vref2 according to the detection result.Further, during the hold period of the SH signal, the sample-and-holdcircuit 62 maintains the detection result and outputs the last variablereference voltage Vref2 as it is.

The variable reference voltage Vref2 from the sample-and-hold circuit 62is applied to a non-inverting input terminal of the error amplifiercircuit 63 and a voltage Vo obtained by dividing the output voltage Voutat the resistor R4 and the resistor R5 is input to an inverting inputterminal of the error amplifier circuit 63. The error amplifier circuit63 is a circuit, which compares the variable reference voltage Vref2 andthe voltage Vo corresponding to the output voltage Vout and outputs anerror signal therebetween. An error voltage between the variablereference voltage Vref2 and the voltage Vo is amplified and output froman output terminal of the error amplifier circuit 63 as the errorsignal. The control circuit 64 drives the MOSFET Q1 to be turned on/offbased on the error signal from the error amplifier circuit 63.

In this way, the output voltage control circuit 6 controls the outputvoltage Vout so that the detected voltage Vmin from the selectioncircuit 4 becomes the reference voltage Vref. Here, the detected voltageVmin from the selection circuit 4 is the lowest voltage of the voltagecorrelated to the current flowing through the MOSFETs Q2 to Q4 in thesample period of the SH signal (the ON-period of the switching elementin which any one of the MOSFETs Q2 to Q4 is driven to be turned on andwhen any of the light emitting element arrays 11 to 13 is turned on).For example, when all of the MOSFETs Q2 to Q4 are turned on, the voltagecorrelated to the smallest current flowing through the MOSFETs Q2 to Q4is the detected voltage Vmin. Further, when the MOSFET Q2 is turned onand the MOSFETs Q3 and Q4 are turned off, the voltage correlated to thecurrent flowing through the MOSFET Q2 is the detected voltage Vmin.Accordingly, in the sample period of the SH signal, the output voltagecontrol circuit 6 controls the output voltage Vout so that the smallestcurrent of the current flowing through the MOSFETs Q2 to Q4 is a presetreference current value. The reference current value is determined bythe reference voltage Vref. Since the current actually flowing throughthe MOSFETs Q2 to Q4 is larger or smaller relative to the referencecurrent value, the reference voltage Vref is set so that the referencecurrent value is slightly larger than a lower limit current value tomake the light emitting element arrays 11 to 13 emit light.

In the present illustrative embodiment, the output voltage controlcircuit 6 is configured to control the output voltage Vout based on theerror signal from the comparison error amplifier circuit 63, whichcompares the variable reference voltage Vref2 with the voltage Vocorresponding to the output voltage Vout. However, the output voltageVout may be controlled based on the variable reference voltage Vref2without using the comparison error amplifier circuit 63. FIG. 5 shows anoutput voltage control circuit 6 a including a control circuit 65, whichcontrols the output voltage Vout based on the variable reference voltageVref2. The output voltage control circuit 6 a is provided with aterminal T7 to which a dimming signal is input from the outside. Thedimming signal for controlling the brightness of the light emitting unit1 by the duty ratio (time ratio) is input to a control terminal of thecontrol circuit 65. The control circuit 65 drives the MOSFET Q1 to beturned on/off based on the variable reference voltage Vref2 when thedimming signal is in an ON-duty state. In addition, the control circuit65 is adapted to stop the on/off drive of the MOSFET Q1 when the dimmingsignal is in an OFF-duty state. As a result, the supply of power to thesmoothing capacitor C1 is stopped when the dimming signal is in theOFF-duty state. Accordingly, it is possible to prevent the supply amountof power to the smoothing capacitor C1 from excessively increasing andthus to prevent the output voltage Vout from unnecessarily increasing.In this method, the supply of power to the smoothing capacitor C1 iscompletely stopped when the dimming signal is in the OFF-duty state.Accordingly, this method is effective in the case where the leakagecurrent from the smoothing capacitor C1 is a negligible value.

As described above, the present illustrative embodiment is directed to alight emitting element driving device that flashing-drive a plurality oflight emitting element arrays 11 to 13 connected in parallel by theswitching elements (MOSFETs Q2 to Q4) connected in series with each ofthe plurality of light emitting element arrays 11 to 13. The lightemitting element driving device includes the current detection resistorsR1 to R3, the selection circuit 4 and the output voltage control circuit6. Each of the current detection resistors R1 to R3 detects the currentflowing through each of the plurality of light emitting element arrays11 to 13 as the current of the light emitting element arrays. Theselection circuit 4 selects the smallest current of the light emittingelement arrays. The output voltage control circuit 6 controls the outputvoltage Vout supplied to the plurality of light emitting element arrays11 to 13 so that the current of the light emitting element arrays 11 to13 selected by the selection circuit 4 becomes a preset referencecurrent value. Accordingly, it is not necessary to use aconstant-current driver in order to flashing-drive the light emittingelement arrays 11 to 13 and thus it is possible to reduce power loss inthe switching elements (MOSFETs Q2 to Q4) that flashing-drive the lightemitting element arrays 11 to 13.

In addition, according to the present illustrative embodiment, the lightemitting element driving device is provided with the average currentcontrol circuit 5. The average current control circuit 5 controls eachof the duty ratios to drive the plurality of the switching elements(MOSFETs Q2 to Q4) to be turned on/off so that the average value of thecurrent of the plurality of light emitting element arrays is equal toeach other. Accordingly, even when the value of the current actuallyflowing through the light emitting element arrays 11 to 13 is differentfrom each other due to variation in the characteristics of the lightemitting element arrays 11 to 13, light can be emitted from the lightemitting element arrays 11 to 13 at the same brightness.

Further, according to the present illustrative embodiment, the outputvoltage control circuit 6 is configured to control the output voltageVout based on the current of the light emitting element arrays, in theON-period of the switching elements in which any one of the plurality ofthe switching element (MOSFETs Q2 to Q4) is driven to be turned on.Then, the output voltage control circuit 6 is configured to control theoutput voltage Vout based on the current of the light emitting elementarrays in the last ON-period of the switching element, in the OFF-periodof the switching elements in which all of the plurality of the switchingelements (MOSFETs Q2 to Q4) are controlled to be turned off. Thereby,even when all of the plurality of the switching elements (MOSFETs Q2 toQ4) are controlled to be turned off and the current of the lightemitting element arrays is not detected, the output voltage Vout can becontrolled on the basis of the current of the light emitting elementarrays in the last ON-period of the switching element. As a result, astable feedback control can be performed.

Further, according to this disclosure, the output voltage controlcircuit 6 is configured to generate the variable reference voltage Vref2based on the current of the light emitting element arrays selected bythe selection circuit 4 and to perform a feedback control of the outputvoltage Vout based on the variable reference voltage Vref2. Thereby,even when all of the plurality of the switching elements (MOSFETs Q2 toQ4) are controlled to be turned off, the output voltage is controlled sothat the variable reference voltage Vref2 and the voltage Vo obtained bydividing the output voltage Vout are equal to each other. Accordingly,it is suppressed that the output voltage Vout is excessively increasedor decreased and therefore a stable feedback control can be performed.

Hereinabove, this disclosure has been explained with reference to thespecific illustrative embodiment. However, the specific illustrativeembodiment is illustrated only as an example, and it is obvious thatvarious modifications can be applied without departing a spirit and ascope of this disclosure.

What is claimed is:
 1. A light emitting element driving device, whichflashing-drive a plurality of light emitting element arrays connected inparallel by switching elements connected in series with each of theplurality of light emitting element arrays, the light emitting elementdriving device comprising: a current detection unit configured to detectrespective currents flowing through each of the plurality of lightemitting element arrays as currents of the light emitting elementarrays; a selection unit configured to select the smallest current ofthe light emitting element arrays of the detected currents obtained bythe current detection unit; and an output voltage control unitconfigured to control output voltage supplied to the plurality of lightemitting element arrays so that the current of the light emittingelement arrays selected by the selection unit becomes a preset referencecurrent value, wherein, in an ON-period of the switching elements inwhich any one of the plurality of the switching elements is driven to beturned on, the output voltage control unit controls the output voltagebased on the current of the light emitting element arrays, and wherein,in an OFF-period of the switching elements in which all of the pluralityof the switching elements are controlled to be turned off, the outputvoltage control unit controls the output voltage based on the current ofthe light emitting element arrays in a last ON-period of the switchingelements.
 2. The light emitting element driving device according toclaim 1, further comprising an average current control unit configuredto control each duty ratio to drive the plurality of the switchingelements to be turned on/off so that the average values of the currentof the plurality of light emitting element arrays detected by thecurrent detection unit are equal to each other.
 3. The light emittingelement driving device according to claim 1, wherein the output voltagecontrol unit is configured to generate a reference voltage correlated tothe current of the light emitting element arrays selected by theselection unit and to perform a feedback control of the output voltagebased on the reference voltage.